Detlef Schooss

Determining the size-dependent structure of ligand-free gold-cluster ions

Ligand-free metal clusters can be prepared over a wide size range, but only in comparatively small amounts. Determining their size-dependent properties has therefore required the development of experimental methods that allow characterization of sample sizes comprising only a few thousand mass-selected particles under well-defined collision-free conditions. In this review, we describe the application of these methods to the geometric structural determination of Au+n and Au−n with n = 3–20. Geometries were assigned by comparing experimental data, primarily from ion-mobility spectrometry and trapped ion electron diffraction, to structural models from quantum chemical calculations.

Structure determination of gold clusters by trapped ion electron diffraction: Au14−–Au19−

The structures of mass-selected gold cluster anions Au(14)(-)-Au(19)(-) have been reinvestigated using an improved low temperature trapped ion electron diffraction experiment. Structures were assigned by comparing experimental with simulated scattering functions using model structures obtained by density functional calculations. Flat three-dimensional structures are found for Au(14)(-) and Au(15)(-), hollow cages for Au(16)(-)-Au(18)(-) and a tetrahedral structure is found for Au(19)(-). For several clusters in this series, our assignments differ distinctly from previous assignments.

Experimental structure determination of silver cluster ions (Agn+,19⩽n⩽79)

The structures of mass selected silver cluster cations Ag19+, Ag38+, Ag55+, Ag59+, Ag75+, and Ag79+ have been probed at a temperature of 100K by trapped ion electron diffraction. The structure assignment is carried out by comparison of the experimental scattering intensity with theoretical scattering functions of calculated candidate structures obtained by density functional theory. For the cluster sizes studied the resulting experimental data are invariably best described by structures based on the icosahedral motif, while closed packed structures can be ruled out.

Electronic photodissociation spectroscopy of Aun−⋅Xe (n=7–11) versus time-dependent density functional theory prediction

Electronic (one-photon) photodepletion spectra were recorded for gold cluster anions complexed with one xenon atom over the photon energy range 2.1-3.4 eV. Clusters were generated by pulsed laser vaporization and probed under collisionless molecular beam conditions. The spectra obtained are highly structured with the narrowest features--assigned to individual electronic transitions--having bandwidths of less than 40 meV. Time-dependent density functional theory predictions of optically allowed transitions for the most stable--planar--isomers of the corresponding bare metal cluster anions are generally consistent with the experimental observation.

Electronic photodissociation spectroscopy of Au4+⋅Arn, n=0–4: Experiment and theory

We report experimental and theoretical studies of the electronic absorption spectra of mass selected Au4+⋅Arn (n=0–4) clusters in the photon energy range of 2.14–3.35 eV. Photodissociation spectra were recorded by monitoring ion depletion upon photon absorption, yielding absolute photodissociation cross sections. The experimental spectra were interpreted by comparing them with calculations of the optical response in the framework of time-dependent density functional theory using cluster structures calculated both by density functional theory (B3-LYP functional) and by ab initio calculations at the RI-MP2 level.

A high resolution dual mass gate for ion separation in laser desorption/ionization time of flight mass spectrometry

A dual mass gate is described which allows for high resolution separation of ions under realistic laser desorption/ionization and matrix assisted laser desorption/ionization time of flight mass spectrometry conditions at 10 kV kinetic energy. The device consists of two interleaved comb units sequentially positioned in the ion beam path. This allows significant improvement in resolution relative to one unit by separation of “low pass” and “high pass” functions. Principal limits as given by mechanical dimensions of the device and electrical properties of the switches are also discussed.

Photodissociation spectroscopy of Ag4+(N2)m, m=0–4

We have determined photodissociation spectra of Ag4+(N2)m=0–4 in the photon energy range from 2.1 to 3.75 eV. The cluster complexes were generated by laser vaporization and their absolute photodissociation cross sections were measured in depletion using a modified reflectron time-of-flight mass spectrometer. The band spectra show significant nonmonotonic variations with increasing N2 coverage. The experimental data are contrasted with quantum chemical calculations of ground and electronically excited states using density functional as well as post-Hartree–Fock methods. Experiment and theory are in qualitative agreement allowing tentative assignment of the depletion spectra and insight into the relevant bonding interactions.

Laser-induced fluorescence of rhodamine 6G cations in the gas phase: a lower bound to the lifetime of the first triplet state.

We have studied the gas-phase laser-induced fluorescence of an ensemble of buffer gas-cooled Rhodamine 6G cations (R6G(+)) stored in a quadrupole ion trap at 90 K. The fluorescence resulting from excitation with continuous-wave 488 nm radiation was observed to disappear almost completely on a time scale of seconds, dependent in detail on the excitation laser fluence. Such decay can be explained by the accumulation of R6G(+) in a dark triplet state. This in turn facilitates the first lifetime determination of the lowest triplet state of free R6G(+) by direct ground-state recovery measurements. A lower bound for the half-life was found to be approximately 2 s. Adding oxygen in a volume fraction of 1% to the buffer gas leads to efficient quenching of the triplet state and correspondingly to complete suppression of the fluorescence intensity decay. Different rare gases were applied as buffers for collisional cooling, but no significant changes in the fluorescence properties were found.

On the Structures of 55‐Atom Transition‐Metal Clusters and Their Relationship to the Crystalline Bulk

Correlation of cluster and bulk structure: Electron-diffraction measurements of homonuclear 55-atom transition-metal cluster anions covering essentially all 3d and 4d elements show only four main structure families. Elements with the same bulk lattice morphology generally have a common cluster structure type. The cluster structure types differ in maximum atomic coordination numbers in analogy to the coordination numbers in the corresponding bulk lattices.

Small tin cluster anions: Transition from quasispherical to prolate structures

The structures and energetics of small tin cluster Sn(n)(-) anions up to n=15 were determined by a combination of density-functional theory and three different experimental methods: Ion mobility spectrometry, trapped ion electron diffraction, and collision induced dissociation. We find compact, quasispherical structures up to n=12. Sn(12)(-) is a slightly distorted hollow icosahedron while Sn(13)(-) to Sn(15)(-) have prolate structures, consisting of merged, hollow, in part incomplete, deltahedral subunits: Sn(13)(-) consists of a face-sharing pentagonal bipyramid and tricapped trigonal bipyramid, Sn(14)(-) comprises a face-sharing dicapped trigonal prism and capped square-antiprism, and Sn(15)(-) consists of two face-sharing tricapped trigonal prisms.